Various systems have been deployed to determine the response of the earth's sub-surface strata to electromagnetic fields for geophysical research. Electromagnetic (EM) surveying, or sounding, techniques can provide valuable insights into the likely hydrocarbon content of subterranean reservoirs. Lately, EM surveying systems and techniques have been receiving increasing interest in commercial applications in the search for oil and gas.
Seismic techniques are often used during oil exploration to identify the existence, location, and extent of reservoirs in subterranean rock strata. During seismic exploration, a sound signal is transmitted to the sub-surface strata where the signal encounters geologic anomalies. The seismic signal is then reflected back to receivers such as hydrophones for sub-sea exploration. The signals thus received are analyzed for the appearance of the sub-sea structures, ideally indicative of the presence of hydrocarbons.
Although seismic surveying is able to identify such structures, the technique is often unable to distinguish between the different possible compositions of pore fluids within them, especially for pore fluids which have similar mechanical properties. In the field of oil exploration, it is necessary to determine whether a previously identified reservoir contains oil or just aqueous pore fluids. To do this, often an exploratory well is drilled to determine the contents of the reservoir. However, this is an expensive process, and one which provides no guarantee of reward.
Thus, while oil-filled and water-filled reservoirs are mechanically similar, they do possess significantly different electrical properties and these properties provide for the possibility of electromagnetic based discrimination testing. Also, seismic techniques are not well adapted to the detection of certain other resistivity contrasts which may be useful in the identification of likely candidates for further hydrocarbon exploration.
A known technique for electromagnetic probing of subterranean rock strata is the passive magneto-telluric (MT) method, as described in GB2390904 to University of Southampton. In such a method, the signal measured by a surface-based electromagnetic detector in response to electromagnetic (EM) fields generated naturally, such as within the earth's upper atmosphere, can provide details about the surrounding subterranean rock strata.
However, for deep-sea surveys, all but those MT signals with periods corresponding to several cycles per hour are screened from the sea floor by the highly conductive seawater. Although long wavelength signals which do penetrate to the sea floor can be used for large scale undersea probing, they do not provide sufficient spatial resolution to examine the electrical properties of the typically relatively small scale subterranean reservoirs. Moreover, since MT surveying relies primarily on horizontally polarized EM fields, it is intrinsically insensitive to thin resistive layers.
Nonetheless, measurements of electrical resistivity beneath the sea floor have traditionally played a crucial role in hydrocarbon exploration and reservoir assessment and development. There are clear advantages to developing non-invasive geophysical methods capable of providing such information from the surface or seafloor. For example, the vast savings that may be realized in terms of avoiding the costs of drilling test wells into structures that do not contain economically recoverable amounts of hydrocarbon would represent a major economic advantage.
In research fields that are not of commercial interest, geophysical methods for mapping subterranean resistivity variations by various forms of EM surveying have been in use for many years. Proposals for finding hydrocarbon reservoirs using such EM surveying have also been made and applications to the direct detection of hydrocarbons using horizontal electric dipole (HED) sources and detectors have proved successful.
Thus, CSEM (Controlled Source Electromagnetics), is a technique of transmitting discrete, very low frequency electromagnetic energy using a current generating source, and an electric dipole. CSEM maps the resistivity contrasts in the subsurface. The method is sensitive to relatively high resistive formations imbedded in a relatively low resistive formation. The frequency range in CSEM is typically between 1/32 Hz to 32 Hz; however, most applications are sub-hertz (less than 1 Hz).
In operation, a fleet of ocean-bottom receivers are deployed. The transmitting source is then towed above these receivers, and the receivers detect the transmitted EM field, which is altered by the presence of the varying resistivity of the subsurface within the range of the receivers.
A typical CSEM surveying system would have a transmitter antenna for use in EM surveying beneath the ocean floor and would include a current source housed in a fish and a dipole antenna. The dipole antenna comprises a first electrode mounted on a cable and located near the current source and a second electrode mounted on a cable and located farther away from the current source. Each electrode is electrically connected to the current source. The transmitter antenna may be deployed by being towed behind a vessel and various sensors may be mounted on each cable.
While such systems show promise in commercial exploration, they suffer from certain drawbacks and limitations. The range of such systems is determined by many factors, including frequency and current of the current source. Such systems have a single source, and thus the frequency and current are established by engineering factors, and cannot be varied outside rather limited parameters. The current state of the art involves using a single current source to output up to about 1600 Amperes. Those of skill in this art generally consider approximately 1700 Amperes to be the physical limit for a single source application, limited primarily by the cross-sectional diameter of current carrying conductors and the state of the art in the effectiveness of insulators.
Furthermore, such systems have a limited number or even just a single transmitter geometry, and are not adapted to scaling by the addition of components in various arrangements.
Thus, there remains a need for an EM system which provides flexibility in the arrangement of plural power sources to increase the effective range and capability of such known systems. The present invention addresses these and other drawbacks of the art.